The present invention relates to a method of producing a solid-state imaging device having a buried getter sink layer formed by introducing a substance of a congener of an element of a semiconductor substrate, such as carbon, to a silicon substrate, on which a crystal growth layer is formed, and a solid-state imaging element is formed in and on the crystal growth layer.
Particularly, the present invention relates to a method of producing a solid-state imaging device wherein white defects are reduced by improving a gettering capability of a buried getter sink layer for gathering impurities which contaminate the crystal growth layer to areas other than element formation regions.
As a semiconductor substrate for being formed a semiconductor element, generally a CZ substrate grown by the CZ (Czochralski) method, an MCZ substrate grown by the MCZ (Magnetic field Czochralski) method, and an epitaxial substrate wherein an epitaxial layer is formed on a surface of the CZ substrate or MCZ substrate, etc. are often used.
Particularly, the epitaxial substrate and the MCZ substrate are mainly used for a solid-state imaging device so as to reduce unevenness of image contrast caused by dopant concentration inhomogeneities (dopant striations). Among the above, the epitaxial substrate indicates a substrate provided in advance with an element formation layer (hereinafter, referred to as an epitaxial layer) formed by crystal growth. The epitaxial substrates are capable of reducing series resistance of a lower part than the epitaxial layer by using a low resistance substrate in its formation and/or by forming a low resistance buried region prior to the crystal growth. Therefore, when using the epitaxial substrate, a drive voltage for bringing a desired change of energy barrier height in an element and an application voltage to a substrate are reduced, which is advantageous to a reduction of power consumption. Accordingly, a solid-state imaging device using an epitaxial substrate is expected to be in wider use in the future.
The Chemical Vapor Deposition (CVD) is used as a practical method for forming a silicon epitaxial substrate. The CVD is performed by the hydrogen reduction method using SiCl4 or SiHCl3 as a source gas, or the pyrolysis method using SiH2Cl2 or SiH4 as a source gas.
Reactions of using the above main four kinds of source gases are indicated below.
(Hydrogen Reduction Method)
SiCl4+2H2xe2x86x92Si+4HClxe2x80x83xe2x80x83(1)
SiHCl3+H2xe2x86x92Si+3HClxe2x80x83xe2x80x83(2)
(Pyrolysis Method)
SiH2Cl2xe2x86x92Si+2HClxe2x80x83xe2x80x83(3)
SiH4xe2x86x92Si+2H2xe2x80x83xe2x80x83(4)
From the above, a substrate formed by using SiHCl3 as a source gas is mainly used for solid-state imaging element because the source gas is inexpensive and suitable to formation of a thick epitaxial layer of a high growing speed, etc.
However, even in a case of forming an epitaxial substrate by using any of the above source gases, a large amount of contaminating impurities, particularly metal impurities or heavy metal impurities, are mixed in during formation of the epitaxial layer. Accordingly, when forming a solid-state imaging element by using the epitaxial substrate, the metal impurities, etc. contained in the substrate cause an increase of a dark-current of the solid-state imaging element and white defects (white spots in the dark) arises much in the solid-state imaging element. Therefore, characteristics and yield thereof decline.
Stainless (SUS) based parts in a bell jar of the epitaxial growth apparatus and piping of a source gas are considered as sources to generate heavy metal impurities. When a chloride (Cl) gas is included in the source gas, the gas is decomposed at the time of epitaxial growing to generate hydrogen chloride (HCl). It is considered that as a result that the HCl corrodes the SUS based parts, metal chlorides are taken into the source gas and further into the epitaxial layer.
Also, prior to forming the epitaxial layer, a HCl gas is intentionally introduced to lightly etch off the surface of the silicone substrate on an object of cleaning the same in some cases, but the HCl is also a part of reasons of corroding the SUS based parts, etc.
Therefore, when forming a solid-state imaging element by using an epitaxial substrate, some kind of gettering technique is necessary for removing metal impurities mixed in as explained above. As the gettering technique, there are the Intrinsic Gettering (IG) by which an oxide of oxygen and silicon included in the silicon substrate is deposited only inside the substrate to be used as a getter sink and the Extrinsic Gettering (EG) by which polycrystalline silicon or concentrated phosphorous (P) regions, etc. is formed on the back surface of the substrate and a getter sink is formed by using a strain stress with silicon. However, neither of the above methods was sufficiently capable as a gettering method and was not able to sufficiently reduce white defects caused by a dark-current of a solid-state imaging element.
To reduce the white defects as above, the present inventors have proposed a technique of, for example, performing ion implantation of a congener element of a silicone substrate, such as carbon, on one surface of the substrate by a doze of 5xc3x971013 ions/cm2 or more and forming an epitaxial layer of silicon on its surface in the Japanese Unexamined Patent Publication No. 6-338507.
According to the method, white defects of a solid-state imaging device is reduced to ⅕ or less comparing with that in the case of an epitaxial substrate using a conventional gettering method.
FIG. 1 shows a reduction of white defects of the solid-state imaging device according to the method described in the Japanese Unexamined Patent Publication No. 6-338507, wherein the number of white defects was standardized by assuming the number of white defects when not performing ion implantation to be 1. As shown in FIG. 1, by performing ion implantation of carbon, white defects widely reduce when a doze of carbon is 5xc3x971013 ions/cm2 or so and further reduce when a doze of carbon is 5xc3x971013 ions/cm2 or more, for example, raised to 5xc3x971014 ions/cm2.
Note that when a doze of carbon exceeds 5xc3x971015 ions/cm2, the crystallinity of a mirror surface of the substrate and a silicon epitaxial layer grown thereon are reduced.
From the above, in the Japanese Unexamined Patent Publication No. 6-338507, there is described that a range of 5xc3x971013 to 5xc3x971015 ions/cm2 is preferable as a doze of carbon.
However, along with the solid-state imaging devices getting highly sensitive, a further reduction of white defects has been desired and a necessity of further improvement of the above conventional method has risen.
The present invention was made in consideration of the above problem and has as an object thereof to provide a method of producing a solid-state imaging device to reduce white defects caused by a dark-current by effectively bringing out an ability of gettering contaminating impurities mixed in during formation of a crystal growth layer for forming a solid-state imaging element, for example, for a buried gettering layer formed by introducing carbon to a silicon substrate.
To attain the above object, according to a first aspect of the present invention, there is provided a method of producing a solid-state imaging device, including the steps of forming a buried getter sink layer by introducing to a semiconductor substrate a substance of a second element which is a congener of a first element composing the semiconductor substrate, forming a crystal growth layer by crystal growing the substance of the first element on a surface of the semiconductor substrate, and forming a solid-state imaging element in and on the crystal growth layer at a lower temperature than that in the case of forming an extrinsic getter sink layer by introducing a substance of a third element of a different group from the first element to a back surface of the semiconductor substrate.
Preferably, a temperature of all processes to form the solid-state imaging element is lower than 1100xc2x0 C.
For example, the first element is silicon and the second element is carbon. Also, the third element is phosphorous.
To attain the above object, according to a second aspect of the present invention, there is provided a method of producing a solid-state imaging device, including the steps of forming a buried getter sink layer by introducing to the semiconductor substrate a substance of a second element which is a congener of a first element composing a semiconductor substrate, forming a crystal growth layer by crystal growing the substance of the first element of a surface of the semiconductor substrate, and forming a solid-state imaging element in and on the crystal growth layer at a lower temperature than that in the case of forming an extrinsic getter sink layer on a surface of the crystal growth layer by oxidization in an atmosphere of a gas containing hydrochloric acid.
Preferably, a temperature of all processes to form the solid-state imaging element is lower than 1100xc2x0 C.
For example, the first element is silicon and the second element is carbon.
In a method of producing a solid-state imaging device as above, a buried getter sink layer is formed by introducing a substance of a congener element to a semiconductor substrate. At this time, for example, carbon C, germanium Ge, tin Sn or lead Pb in the same group IV, for example, ion implanted to, for example, a silicon substrate. In the case of carbon, residual oxygen in the silicon substrate bonds with carbon to generate a compound, which becomes a getter sink. Next, for example, a crystal growth layer of n-type silicon Si is stacked on the semiconductor substrate. Then, the crystal growth layer is formed a variety of impurity regions, and a transfer electrode, light blocking film, filter or lens, etc. are formed on the upper surface of the crystal growth layer, so that a solid-state imaging element is completed.
During thermal processing in a process of forming the solid-state imaging element, for example, heavy metal impurities mixed in at the time of stacking the crystal growth layer are drawn to the above getter sink, consequently, the contamination degree of the crystal growth layer on which the element is formed is lowered.
The present inventors found from experiments that the gettering effect actually declined when a temperature in the processes after forming the crystal growth layer was set too high. Specifically, they obtained a new knowledge that in addition to forming the buried getter sink layer by introducing carbon, etc., when forming other external getter sink layer requiring high temperature processing in parallel therewith, it inhibits the effect of the buried getter sink layer and the gettering effects were not sufficiently obtained as a whole. For example, both of the formation of an external getter sink layer by performing phosphorous diffusion on the back surface of the substrate and formation of an external getter sink layer by performing high temperature oxidization on the surface of the substrate in an atmosphere of chloride contained gas require high temperature processing of 1100xc2x0 C., and when not forming the external getter sink layer, the gettering effects improve. From the above, in order to sufficiently bring out the effects of the buried getter sink layer by carbon introduction, there was obtained new knowledge that a temperature of processes thereafter had an upper limit.
A method of producing a solid-state imaging device according to the present invention is invented based on the knowledge as above and characterized by forming a solid-state imaging element by setting the temperature lower than that at the time of forming the extrinsic getter sink layer by introducing a substance of a third element of a different group from the above first element to the back surface of the semiconductor substrate, or by setting the temperature lower than that in the case of forming the extrinsic getter sink layer on the surface of the crystal growth layer by oxidizing in an atmosphere of a chloride contained gas. As a result, the gettering effect by introducing a congener element of the semiconductor substrate (for example, carbon) is sufficiently brought out and the contamination degree of the crystal growth layer is furthermore lowered.